Recently, noticeable developments have taken place in the technologies of converting image data into electric signals and transmitting and storing data and of changing the layout or color tone of images and reproducing them on CRTs. With the development, the demand for hard copies from such image data is significantly increasing, and various hard copy means have been proposed. However, many of these means have a low image quality and, in particular, almost all color hard copies produced by such means are not good and are not comparable to prints made using the current generation of color papers. As one example of providing high-quality hard copies, there is Pictorography (trade name of Fuji Photo Film Co., Ltd.), which utilizes a silver halide heat-development dye-diffusion system and an LED scanning exposure system.
On the other hand, with development of the technology of silver halide photographic materials and a compact simple rapid development system for processing such materials (for example, mini-laboratory systems), photographic prints of extremely high quality can be provided easily and inexpensively in a short period of time. Regarding hard copies of stored images, the demand for high-quality hard copying materials which are inexpensive and which may be processed simply and rapidly to give stable and high-quality hard copies is extremely great.
As a system of obtaining hard copies from electric signals by the use of a silver halide photographic material, in general, a scanning exposure system of exposing the material while successively taking out image data from electric signals is employed, and a photographic material suitable for use in the system is needed. For the purpose of rapidly obtaining hard copies by the use of a silver halide photographic material, it is necessary to shorten both the time for scanning exposure and the time for development.
Various practical recording devices for scanning exposure are known. As light sources for recording with the devices, glow lamps, xenon lamps, mercury lamps, tungsten lamps and light emitting diodes and the like have heretofore been employed. However, all such light sources have such drawbacks for practical use that the output power is weak and the life is short so that they are not suitable to achieve the object of rapid scanning exposure. Scanning exposure devices capable of compensating for these drawbacks are known, in which coherent laser ray sources, for example, gas lasers such as a He--Ne laser, an argon laser or a He--Cd layer, semiconductor lasers, solid lasers, or secondary harmonic lasers to be obtained by combining such laser ray sources and non-linear optical materials are used as light sources for scanning exposure.
Gas lasers may yield a high output power but have the drawback that a large-scale and expensive device is needed.
As opposed to them, semiconductor lasers have such advantages that a small-size and inexpensive device may be employed, that a modulator is not needed since direct modulation is possible and that the life of semiconductor lasers is longer than that of gas lasers.
Because of these reasons, semiconductor lasers are often used in scanning exposure systems for printing photographic materials. The exposure device for the system is for dot exposure, and the modulation signals for it are those of binary information which may be controlled by an on-off change of a certain constant quantity of light. In the device, therefore, the minimum modulation time per pixel may be controlled within the range of about 20 ns. However, where an image with gradation like a hard copy is to be formed on a support, the quantity of light must be modulated in plural stages (of at least 6 bits or more, preferably 8 bits or more) so as to obtain a satisfactory image quality. The modulation methods for semiconductor lasers are grouped into an intensity modulation system where the current for the laser is changed to change the light intensity, and a pulse width modulation system where the exposure time per pixel is changed to change the quantity of light with the light intensity of the laser being constant. The two systems may be employed singly or in combinations of them. In the intensity modulation system, since the light intensity of the laser is varied, the quantity of heat to be generated is varied in accordance with the amount of exposure. Therefore, the light intensity can hardly be controlled, as compared with the pulse width modulation system. In addition, the controllable minimum time per pixel is longer in the intensity modulation system than in the pulse width modulation system. On the other hand, it is difficult to shorten the exposure time per pixel to less than several hundred nanoseconds in the pulse width modulation system because of the problem of the stability of modulation, at present. Where an A-4 size (210 mm.times.297 mm) hard copy is desired to be exposed with an image density of 400 dpi, exposure of about 15,000,000 pixels is needed. In such a case, even if one pixel is exposed in 5.times.10.sup.-7 sec, the exposure of the hard copy of the A-4 size requires about 8 seconds, which is a great bar to the elevation of the copying speed.
Recently, due to advances in the technology of external modulators, waveguide acousto-optical modulators or waveguide electro-optical modulators have been developed, with which modulation at a rate of at most several ns per pixel has come to be possible. However, it has been found that application of such extreme short exposure to a silver halide photographic material causes differences in the density and the color tone between pictorial image areas with gradual density variation and image areas composed of fine lines such as computer graphics or letters, which cannot be observed in conventional scanning exposure.
On the other hand, if development is desired to be effected simply and rapidly, use of a silver halide emulsion having a high silver chloride content, such as that described in U.S. Pat. No. 4,892,804, is indispensable. However, it has been found that such a silver halide emulsion having a high silver chloride content brings about much more increase of the above-mentioned difference than a silver chlorobromide emulsion having a low silver chloride content or than a silver bromide emulsion. In addition, if the time for development is desired to be shortened, using an emulsion having a high silver chloride content, the difference increases even more.
Therefore, in order to obtain hard copies of constant quality simply and rapidly, by exposing a photographic material having a silver halide emulsion having a high silver chloride content with an exposing device equipped with the above-mentioned high-speed modulator, it is necessary to develop a silver halide photographic material which hardly causes differences in the density and the color tone between the pictorial image area and the image area of letters and fine lines due to differences between the objective scenes.